1. Field of Invention
This invention relates generally to a lithographic system for pattern generation. More specifically it relates to a lithographic system having a raster scanned, Gaussian beam writing strategy for exposing a pattern.
2. Description of the Related Art
Lithographic systems typically generate or expose patterns by controlling the flow of energy from a source to a substrate coated with a layer sensitive to that form of energy. Pattern exposure is controlled and broken into discrete units commonly referred to as flashes, wherein a flash is that portion of the pattern exposed during one cycle of an exposure sequence. Flashes are produced by allowing energy from the source, for example light, electron or other particle beams, to reach the coated substrate within selected pattern areas. The details of flash composition, dose and exposure sequence used to produce a pattern, and hence the control of the lithographic system, make up what is known as a writing strategy.
Writing strategies strive for both the highest pattern throughput and best pattern quality. However, often highest throughput is only possible at the cost of degraded pattern quality. For example, smaller flashes usually result in better pattern quality but lower throughput. Thus an optimized writing strategy is one that makes the best compromise for each specific task. Both vector scan and raster scan writing strategies strive for the same goals, therefore a combination of elements of each may result in a better compromise.
A traditional raster scan writing strategy employs a uniform periodic raster scan, much like a television. A mechanical stage moves a substrate uniformly in a direction orthogonal to the direction of the uniform scan of an energy beam. In this manner a pattern is composed on a regular grid with a regular scan trajectory resulting from the orthogonal movement of the stage and beam. When the beam is positioned over a grid site requiring exposure, the beam is unblanked and the underlying site exposed. Only the amount of dose, or energy, at each site is varied as required. Hence, exposure data can be organized in a time sequence corresponding to the regular scan trajectory, and only the dose for each site need be specified. The distinguishing characteristics of a traditional raster scan writing strategy are a small round beam exposing one site at a time, a periodic scan moving sequentially to each site of a grid and a rasterized representation of data corresponding to the required dose for each site or "pixel" of the grid. See, for example, U.S. Pat. No. 5,393,987, to Abboud et al., entitled DOSE MODULATION AND PIXEL DEFLECTION FOR RASTER SCAN LITHOGRAPHY, column 1, line 5 to column 2, line 12 which is incorporated herein by reference in its entirety (hereinafter "Abboud"). In addition, raster scanning has an inherent asymmetry between scan and stage directions, despite a symmetric Gaussian distribution of energy within the round beam. Thus, in cross-section, an energy profile deposited in a resist layer will display a different slope to the edge of the exposed region in the scan direction than in the stage direction. This difference in slope, often results in differences in critical dimensions of like sized features measured in scan direction versus the stage direction.
On the other hand, in a typical vector scan writing strategy, the beam is positioned only over those sites that require exposure and then unblanked to expose the site. Positioning is accomplished by a combination of stage and beam movement in what is often referred to as a semi-random scan. Thus, data must be provided that includes both the dose and position of each flash or site exposed. Frequently, vector scan strategies use a variable shaped beam, that is a beam capable of having a different size and/or shape for each flash. The pattern is then composed from these variable shapes. A shaped beam is capable of exposing multiple pixel sites simultaneously instead of one pixel site at a time as in a raster scan writing strategy. Where a variable shaped beam is used, the data must additionally include the location, size and shape for each flash. Thus the distinguishing characteristics of traditional vector scan writing strategies are a variable shaped and sized beam exposing multiple pixel sites in a single flash, a semi-random scan encompassing only those portions of a pattern to be exposed, and a vectorized representation of data including the location, size, shape and dose of each flash.
Important to both raster and vector scan writing strategies is the pattern coverage rate, R specifying the pattern area exposed per second of writing time. R is normally expressed having the dimensions of square centimeters per second (cm.sup.2 /sec). Both writing strategies strive to have a high R. High coverage rates imply high flash rates, while pattern integrity or quality implies that small pixels be used to define pattern shapes. Thus with a limited flash rate, optimization of a writing strategy favors exposure of as many pixels as possible during each flash.
As known, flash rate (F) in Hertz (Hz) and energy or flux density (J) expressed in Amperes per square centimeter (Amp/cm.sup.2), are limited by both electronics and the beam optics. The relationship between R, F and J can be expressed by looking at a lithography system that exposes a pattern on a substrate having an energy sensitive layer or resist requiring an amount of energy or dose D expressed in microCoulombs per square centimeter (.mu.C/cm.sup.2) and that uses N separate beams in p separate exposure passes. We define .DELTA. to be an address unit, or the period of a grid upon which the pattern is composed. Each element of the grid is called an "address element" which covers an area .DELTA..sup.2 (cm.sup.2). If each flash can expose an average of n.sub.x address elements along the x direction and n.sub.y address elements along the y direction, and requires one flash period 1/F (sec) to expose the flash, the coverage rate is seen as subject to the following two constraints:
R&lt;N n.sub.x n.sub.y.DELTA..sup.2 F/p EQU R&lt;N n.sub.x n.sub.y.DELTA..sup.2 J/D
which implies a current density requirement of J=DF/p
The size .DELTA. of an address element typically determines pattern placement precision. The size of a beam used to expose an address element usually determines pattern edge resolution and control of critical feature sizes. As known, beam size must be at least as large as an address element, therefore pattern quality considerations limit pixel size. With these limitations, it is seen that writing strategies should strive to maximize the number of address elements exposed during each flash.
Both vector and raster scan writing strategies have advantages and disadvantages. Vector scan strategies can often write patterns faster because larger pieces of the pattern are exposed in each flash using shaped beams. In addition, vector cans strategies can offer arbitrarily fine placement precision (as distinguished from accuracy) by adding least significant bits to digital to analog converters (DACs) used for beam deflection. However, the semi-random scan trajectory characteristic of a vector scan strategy usually requires several levels of precision DAC driven-electronics that must be fast, stable and well calibrated to avoid "butting" or "stitching" errors between deflection fields. Such electronics are sophisticated hence add to system cost and complexity. Also, vector scan flash rates are typically slower due to settling time required between the relatively large beam deflections of the semi-random scan trajectory. In addition, where beam shaping is employed, dose errors due to shaped beam size and shape variations can be generated. Finally, since vector scan systems usually spend more time exposing small deflection fields before moving on to other areas of the pattern, heating of the resist is more localized an thus is a larger threat to pattern quality.
Raster scan strategies are relatively simple and accurate because a minimum number of periodic deflections (stage motion and scan) are used to position the beam. However, since a single beam exposes one pixel at a time in a serial manner, raster scan strategies tend to have a low coverage rate and/or a relatively coarse address grid. In addition, as previously mentioned, differences in the slope of the edges of energy profiles in the scan and stage directions can lead to differences in critical dimensions (CD) as measured for features in each direction. Stretching the beam shape in the stage direction, that is forming a slightly astigmatic beam, can mitigate these CD differences by shallowing the edge profile. However, as edge placement is a function of edge slope, the shallower slope reduces edge placement control as compared to non-astigmatic beams. In a similar manner, beam defocusing, another method found to mitigate CD differences, also reduces edge placement control. In still another method for reducing this CD difference, beam un-blanking is delayed and blanking advanced. In this manner each exposure is compressed in the scan direction. However, this method has the deficiency of introducing butting error even where the stripes (See, Collier, FIG. 1, #30) perfectly butt against one another. The butting error appears as a reduction in dose at the stripe butt for any figure formed from writing within adjacent stripes.
Thus it would be desirable to develop an improved writing strategy that combines the advantages of a vector scan strategy with those of a raster scan strategy. It would also be desirable to develop an improved writing strategy that made the aforementioned combination using a rasterized representation of the pattern for exposure. In addition, it would also be desirable to reduce or eliminate CD variations in the scan and stage directions, respectively. Finally it would also be desirable to develop the an improved writing strategy capable of using methods for the correction of proximity effects during run-time.